Capacitor-discharge stud welding apparatus

ABSTRACT

In the stud welding apparatus disclosed a battery or bank of capacitors in the main welding line stores welding energy. A current supply unit charges the bank of capacitors. An electronic switch applies the energy from the bank of capacitors to welding electrodes. A control device, which can be turned on and off, renders the electronic switch conductive when it is turned on. A safety circuit reduces the output voltage of the current supply unit when the electronic switch happens to turn on while the control device is turned off. The safety circuit also reduces the voltage across the bank of capacitors.

United States Patent [1 1 Harder CAPACITOR-DISCHARGE STUD WELDINGAPPARATUS [75] inventor: Karl-Max Harder, Vaduz,

' Liechtenstein [73] Assignee: Hilti Aktiengesellschait, Schaan,Liechtenstein [22] Filed: Apr. 6, 1972 211 Appl. No.: 241,667

[30] Foreign Application Priority Data 3,355,570 11/1967 Glorioso..2l9/98 1 Oct. 23, 1973 Primary Examiner-R. F. Staubly Attorney-DavidToren et a1.

[57] ABSTRACT In the stud welding apparatus disclosed a battery or bankof capacitors in the main welding line stores welding energy. A currentsupply unit charges the bank of capacitors. An electronic switch appliesthe energy from the bank of capacitors to welding electrodes. A controldevice, which can be turned on and off, renders the electronic switchconductive when it is turned on. A safety circuit reduces the outputvoltage of the current supply unit when the electronic switch happens toturn on while the control device is turned off. The safety circuit alsoreduces the voltage across the bank of capacitors.

29 Claims, 6 Drawing Figures PATENTEDUBI 23 I975 3,767,887

SHEET 3 [IF 3 particular stud welding apparatuses which BACKGROUND OFTHE INVENTION This invention relates to welding apparatuses, and independ upon capacitor discharges.

In such apparatuses a bank or battery of capacitors in the main weldingline stores the welding energy. A current supply unit charges thecapacitor bank. An

electronic switch in the main welding line controls flow of energy fromthe capacitor battery to the welding electrodes, and a control devicecontrols the state of the electronic switch.

Electronic switching elements, particularly semiconductor switchingelements occasionally become conductive accidentally without beingstarted. At other times they fail to return to their non-conductivestate even when the normal operating voltages would render themnon-conductive. Uri-intentional triggering may result from accidentalelectrostatic charges or the inductance of welding apparatuses in whichsuch electronic switching elements are used. On the other hand,excessive current surges, such as may be encountered in weldingapparatuses, may be responsible for failure of the switching elements toreturn to their blocked or non-conductive state.

In stud welding apparatuses of the above-mentioned type, a thyristor orplurality of thyristors are used as electonic switches for controllingthe appropriate welding circuit. In such welding apparatuses measuresare usually taken to make sure that the welding current can beinitialled only when the mounting for the welding electrodes orelements, or the Welding electrodes or elements themselves, are properlyplaced on the workpiece. This avoids any danger to the operator when thethe current is turned on. However, if the thyristor or plurality ofthyristors which control the welding current have failed to return tothe non-conductive or blocked state after carrying a welding current,the voltage of the current source or the voltage of the battery ofcapacitors remains between the welding electrodes or terminals.Depending upon the welding circuits used, such a voltage can reachseveral hundred volts. This type of voltage represents a substantialhazard for the operator and may result in serious accidents.

Moreover, stud welding apparatuses frequently do not remain stationarywhile in use. For example, in as sembly operations they are composed ofa hand-guided welding gun, a largely portable device for generating thewelding current, and the necessary connecting cables. Such assemblyoperations frequently involve work conditions which are intrinsicallyhazardous. The unexpected appearance of a voltage between the weldingelectrodes, even if the voltage is not inherently dangerous, wouldincrease the likelihood of accidents and thus magnify the hazard.However, within reasonable expenditure limits, it is difficult toprotect operators from contact with welding electrodes.

In order to avoid such a substantial hazard, care must be taken to avoidthe unintentional generation of directly or indirectly hazardousvoltages between the electrodes of a stud welding apparatus. Inparticular, it is important to prevent such hazardous voltages fromappearing when the control device controlling the switching element isnot turned on. On the other hand,

if such unintentional voltages appear it is important that they beeliminated as soon as possible.

SUMMARY OF THE INVENTION According to a feature of the invention, thesedisadvantages are overcome by providing circuit means responsive to theelectronic switch being in the conductive state and the control devicenot being turned on, for reducing the output voltage of the currentsupply unit.

According to another feature of the invention, the circuit means alsoreduces the voltage at the battery of capacitors. The circuit means mayalso be protective means.

According to still another feature of the invention, the circuit meansentirely eliminates one or both of these voltages.

If the electronic switch fails to return to its nonconductive or blockedstate after the conductive device has been turned off, it is sufficientmerely to reduce or cut off the voltage of the current supply unit. Thecapacitor battery, that is, the battery of capacitors, could then nolonger charge, or could only charge to a harmless voltage. Thus,accident hazards would be avoided. However, if the electonic switchbecomes conductive during the charging of the battery of capacitors,cutoff or reduction of the output voltage of the capacitorchargingcurrent supply would not afford sufficient security because the batteryor bank capacitors could be charged to some degree. Thus, a hazardousvoltage might already be formed between the welding electrodes. For thisreason, the voltage at the capacitor bank is reduced or eliminated assoon as possible.

According to another feature of the invention, the circuit means forms ashort circuit across the current supply unit through the electronicswitch when the control device is not turned on.

According to another feature of the invention, the control meansincludes a cutoff device which turns off the electronic switch duringflow of short circuit current.

According to another feature of the invention, the contol deviceincludes a relay which closes the short circuit in response to thecontrol device being turned off. Normally, the electronic switch whichforms part of the short circuit path, interrupts the short circuitcurrent by its usual cutoff. If the electronic switch then becomesconductive or remains conductive, when the control device is no longerturned on, the short circuit current to which the cutoff deviceresponds, flows immediately. The output voltage of the current supply isthus rapidly disconnected.

According to another feature of the invention, the circuit means has anauxiliary switch connected across the electrodes. The auxiliary switchis opened when the control device is turned on and closed when thecontrol device is turned off. The welding electrodes are thus alwaysshort circuited when no welding current is to flow. Should theelectronic switch then become unintentionally conductive, no hazardousvoltage appears between the welding electrodes. At the same time, thecapacitor battery is discharged and the cutoff device responds.

According to another feature of the invention, the auxiliary switch is athyristor or a mechanical switch controlled by a contactor.

According to another feature of the invention, a disconnector isarranged in series with the electrodes in the weldinglines.Thedisconnector is closed only when weldingcurrent is to flow. Thedisconnector would have to carry the entire welding current during thewelding operation. lts contacts could be damaged and become sticky as aresult of such heavy current flow. Thus, it would'not be possible torely upon opening of the disconnector while a welding current flows oruntil the current has been cut off.

According to another feature of the invention, the circuit meansincludes a switching mechanism with a switch that can be released by a'voltage sensor. The switch of the switching mechanism is connected intothe short circuit path which contains the current supply unitand theelectronic switch. A cutoff device which responds when a short circuitcurrent flows is provided for the current supply unit. The switchingmechanism is only effective if the control device is'not turned on. Thevoltage sensor for releasing the switch responds only when theelectronic switch is conductive.

The short circuit path which is exposed to high thermal and mechanicalstresses is kept very small and laid out to withstand these stresses.

According to another feature of the invention, the switching mechanismincludes a thyristor whose control electrode is connected to a voltagesensor in the form of a voltage divider circuit arranged parallel to theseries connection composed of the electronic switch and the capacitorbattery. The voltage sensor can, of course, assume other forms as isknown in the art.

According to another feature of the invention, a current sensor is used.

According to another feature of the invention, the cutoff device is inthe form of a fuse. Preferably, a quick action safety fuse is usedbecause it can effect an extremely rapid dicconnection. Such a fuse canbe readily replaced at little disconnection. and requires very'littlespace while weighing very little. Mechanical switch contacts, on theother hand, would be very complicated, large and heavy if they are notto be destroyed while breaking of the short circuit current.

According to another feature of the invention, the circuit means formsan auxiliary discharge circuit when the control deviceis not turned on.The auxiliary discharge circuit contains the capacitor battery, theelectronic switch, and a discharge resistance. The discharge circuitallows rapid elimination of the voltage across the battery. This isimportant because the battery of capacitors may already be charged whenthe electronic switch inadvertently becomes conductive.

According to another feature of the invention, an additional set ofcontacts of the above mentioned relay form apart of the dischargecircuit for short circuiting the supply current unit. The resistancevalue of the discharge resistance is selected so that the voltage of thecapacitor battery is reduced to a harmless value at a rapid rate, butwithout unduly stressing the switching elements in the dischargecircuit.

According to yet another feature of the invention, the dischargeresistance is preferably the charging resistor of the charging circuitof the capacitor battery. By using the available charging resistor as adischarge resistance, it is possible to save one resistor.

According to another feature of the invention, a switch replacing theelectronic switch is arranged in the discharge circuit of the capacitorbattery. In this way,

when the voltage of the capacitor battery exceeds a predeterminedmaximum permissible value. Such circuit'means reduces the risk ofallowing the electronic switch accidently to pass into the conductivestate.

According to yetanother feature of the invention,

, the current supply unit includes built-in means for current limiting.Preferably the current supply unit includes a transformer with loosecoupling. According to a feature of the invention, such a transformer isa leakage reactance transformer. A transformer of this type limits thecurrents which can flow in the secondary winding. At the same time thevoltage across the secondary winding is substantially reduced when thesecondary winding of the transformer is subjected to short circuitingover the discharge circuit, such as would occur if the electronic switchshould accidently be come conductive; Under these circumstances, it isno longer necessary to cut off the charging current supply unit inresponse to failure of the electronic switch. At the same time, it ispossible to use relatively small switching elements in the safetycircuit.

These and other features of the invention are pointed out in the claims.Other objects and advantages of the invention will become obvious fromthe following detailed description when read in light of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a schematic circuit diagram illustrating a stud weldingapparatus with a voltage protecting circuit and embodying features ofthe invention.

FIG. 2 is a schematic circuit diagram illustrating another embodiment ofthe invention wherein the transformer of FIG. 1 is replaced with acurrent-limiting transformer.

FIG. 3 is a schematic circuit diagram illustrating another apparatusembodying features of the invention.

FIG. 4 is aschematic circuit diagram of yet another apparatus embodyingfeatures of the invention, wherein the transformer according to FIG. 2is replaced substantially by a current-limiting transformer.

HO. 5 is another schematic diagram illustrating an apparatus embodyingfeatures of the invention, which embodiment is similar to that of FIG. 3but where overload protection is provided.

FIG. 6 is a schematic diagram of yet another apparatus embodyingfeatures of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, atransformer 1' receives power from an alternating current line andenergizes a rectifier bridge 6. The bridge 6 charges a capacitor battery1 l, i.e., a bank or battery of capacitors, through a fuse 2, anarmature al, a contact a4, and a series charging resistance 3.

The armature a1 and contact a4 form part of a relay 15 which is shown inits normally unenergized state. When the relay 15 assumes the normallyunenergized position shown, the armature a1 rests against the contacta4.

The capacitor battery serves to furnish a welding current to a weldinggun 16. The welding gun includes a welding electrode 16a which ispressed against a second electrode 17 formed of the material to which astud is to be welded. The capacitor battery 11 produces the weldingcurrent by discharging from its positive terminal 11b, through athyristor 10, a connecting terminal or plug x, the electrode 16a, theelectrode 17, a connecting terminal or plug y, a line 3a, and a diode 21to a negative terminal 11a.

The thyristor 10 forms an electronic welding switch. A resistor 20between the cathode of the thyristor l and a line 3a forms an auxiliaryload for shunting normal residual or leakage currents through thethyristor 10 when the thyristor is in its non-conductive state. Thisprevents buildup of voltage between the welding electrodes due to theleakage of current through the thyristor 10.

A control device 18 for igniting and extinguishing the thyristor isconnected to the control electrode of the thyristor 10. The controldevice 18 constitutes a triggering and extinguishing system. An armaturea2 of the relay 15 can turn the control device 18 on or off, that is toa connected or disconnected state, by switching the armature a2 betweentwo contacts a and a6.

An armature a3 of relay l5 rests against a contact a7 ,when the relay isunenergized. The armature a3 and contact a7 connect the cathode ofthyristor 10 to the negative output of the rectifier bridge 6 to whichthe charging resistance 3 is also connected. A current source Sinitiates the welding current by energizing the relay through the switch19, the attached welding electrode 16b, and the base material 17. Therelay 15 can trip only when switch 19 is closed and when the legs of thewelding gun 16 are electrically connected with the workpiece l7.

For welding the welding element or stud of the gun 16 must alsoelectrically contact the base material 17 so as to close the weldingcircuit.

The circuit of FIG. 1 operates as follows.

The bridge 6 charges the capacitor battery 11 through the elements 2,a1, a4, and 3. To start welding the electrodes 16a and 16b are placedinto contact with the base material 17. The switch 19 is then closed.

Closing of the switch 19 energizes the relay 15 and draws the armaturesa3 and all away from the contacts a7 and a4. This stops charging of thecapacitor battery 11. Energization of the relay 15 also switches thearmature a2 from the contact a5 to the contact a6. This turns on thecontrol device and ignites the thyristor 10. Welding current then flowsfrom the capacitor battery 11 through the thyristor 10, the electrode16a, the base material 17, and a diode 21 in the line 3a of the weldingpath.

The diode 21 prevents the bridge 6 from charging a previously dischargedcapacitor battery 11 through the fuse 2, armature a1, contact a4,terminal 11a, resistor 20, armature a3, and contact a7, when the relay15 is unenergized. It prevents the bridge voltage from appearing acrossthe welding electrodes.

After the switch 19 is open, the relay 15 drops out again. That is tosay, it becomes unenergized. The armatures a1 and a3 then contact thecontact a4 and a7 while the armature a2 switches over to contact a5. Thelatter action turns off the control device 18 and restores the thyristor10 to its non-conductive state. At the same time, the capacitor batteryis connected across the bridge 6 through the fuse 2, the armature al,the contact a4, and the resistance 3.

Under certain circumstances the thyristor 10 does not return to itsnon-conductive state despite its being switched off by the controldevice. On the other hand, the thyristor 10 may accidently becomeconductive any time after it is switched off when the capacitor batteryis charged to some degree. In these cases the positive pole of thebridge 6 becomes short circuited through the fuse 2, the armature al,the contact a4, the conducting thyristor 10, the armature a3, thecontact a7, the negative pole of the rectifier bridge 6. Depending uponthe character of the fuse, it will react more or less spontaneously tothe resulting short circuit by blowing out or disconnecting.

At the same time, the battery 11 is shunted from the terminal 11b to theterminal 11a by the conducting thyristor 10, the armature a3, thecontact 117, and the charging resistance 3. Thus, battery 11 dischargesrapidly over the resistance 3. Consequently, no harmful voltage to theoperator can form between welding plugs x and y when thyristor 10accidently becomes conductive. The supply current is shunted andinterrupted and the capacitorbattery is discharged.

The embodiment of FIG. 2 is very similar to the embodiment of FIG. 1.However, in contrast to FIG. 1, FIG. 2 includes the transformer 1a inplace of the transformer l. The transformer 1a is a current-limitingtransformer, specifically a leakage reactance transformer. Also, in FIG.2, the line 3a is connected directly to an output terminal of therectifier bridge circuit 6 and a discharge resistance 3b appears in theline between the armature a3 and the cathode of thyristor 10.

By virtue of this construction the transformer la can deliver currentswhose values never exceed a predeter- I mined maximum. For this reason,it is possible to eliminate the charging resistance 3 in the chargingcircuit of the capacitor battery 11. One of the particular advantages ofthis circuit lies in the fact that, as the capacitor battery starts tocharge, the limited maximum current of the transformer slows the voltagerise. This increases the time available to permit the switchingmechanism to become operative. This is important when, for example, thefuse 2 at the rectifier bridge is in the form of a relay fuse. Theadditional time may be sufficient to allow such a relay fuse to operate.Such a relay fuse can, for example, disconnect the transformer from themain.

Thus, according to the embodiment of the invention the fuse 2 is in theform of a relay fuse which opens contacts, not shown, between theprimary of transformer 1a and the source, main or supply line.

The embodiment of the invention illustrated in FIG. 3 utilizes a'voltagesensor between the welding lines. In contrast to the embodiment of FIG.1, a short circuit path controlled by a thyristor 5 shunts the rectifierbridge 6 and the fuse 2. The resistor 20 of FIG. 1 is replaced by avoltage divider circuit having resistances 12 and 14 and a diode 13. Thevoltage appearing at the junction of the resistance 12 and the diode 13is applied to the control terminal through the armature a3 and contacta7 of relay 15 to control the thyristor 5. The diode l3 prevents currentfrom surging to the capacitor battery 11 when it is completelydischarged, and the charging current is switched on. Such a currentwould otherwise flow from the bridge 6, through the fuse 2 through thearmature a1, contact a4, the capacitor 11, the resistor 14, the armaturea3, the contact a7, and the control electrodes of thyristor to thenegative terminal of the bridge 6. Such a current surge would turn onthe thyristor 5. The diode 13 prevents the thyristor 5 from being turnedon in this undesired manner.

If the thyristor works properly, the method of operation according toFIG. 3 corresponds exactly to that of FIG. 1. However, if the thyristor10 remains either conductive after switch 19 has been opened, or becomesconductive while switch 19 is open, a voltage appears at the junction ofresistor l2 and diode 13 in the voltage divider. The armature a3contacting the contact a7 applies this voltage to the control electrodeof the thyristor 5. This renders the thyristor 5 conductive.

The thyristor 5 then forms a short circuit across the bridge and thefuse 2.

Theshort circuit current causes fuse 2 to respond. This interrupts thecharging circuit of the capacitor battery 11. In addition, the thyristor5 discharges the capacitor battery 11 by shunting current through thecontact a4, the armature a1, and the resistance 3.

This embodiment makes it impossible for a dangerous voltage to appearbetween the welding electrodes and at plugs x and y while the electrodesare not connected with each other electrically. If the thyristor 10should accidently become conductive any resulting voltage is immediatelyeliminated.

The circuit of FIG. 3 when compared to the circuits of FIGS. land 2, hasthe advantage that the elements a1 and a4 need not carry very high shortcircuit currents before the fuse 2 burns out.

Another embodiment is illustrated in FIG. 4. The latter uses theadvantages of the current-limiting transformer la. This embodiment issimilar to FIG. 3. However, the anode of thyristor 5 here is connecteddirectly to the terminal 11b of the capacitor battery 11. It isconnected between the cathode of thyristor 5 in the negative'terminal ofthe rectifier bridge 6. In FIG. 4, the current flow through the elementsa1 and a4 is limited to a relatively small charging current. This isparticularly important for preventing damage to the armature a1 and thecontact a4. The discharge currents of the capacitor battery 11, which isnecessary for operation of the safety circuit, flows through thethyristor 5 and discharge resistance 3 exclusively. This dischargecircuit need carry only the emergency discharge current and not theregular discharge current. Thus, resistance 3 may be made smaller thanif it had to carry the welding current. .In addition to these structuraladvantages, the circuit of FIG. 4 discharges the capacitor battery veryrapidly. FIG. 5 illustrates an embodiment which combines a safetycircuit with an overload protection system for the capacitor battery.For this purpose, a voltage divider circuit composed of resistors 4 and7 shunts the capacitor battery 11. The voltage divider appears betweenthe resistance 3 and the terminal 11b. The resistor 7-is in the form ofa potentiometer. In addition, a diode 8 and Zener diode 9 carry thevoltage appearing at the tap of potentiometer type resistor 7 to thecontrol electrode of the thyristor 5. The circuit of FIG. 5 otherwisecorresponds to the circuit of FIG. 3.

The circuit of FIG. 5 also operates in a manner similar to that of FIG.3. However, here the thyristor 5 receives its control voltage from thevoltage divider composed of resistors 4 and 7 through the diode 8 andZener diode 9. The thyristor 5 is ignited when the voltage at the tap ofresistor 7 is sufficiently high to cause the voltage across the Zenerdiode to exceed its breakdown potential. The breakdown voltage of theZener diode and the setting of the voltage divider are chosen so thatthe Zener diode breaks down when the charging voltage of the capacitorbattery 11 attains a predetermined maximum admissible value.

This circuit affords not only the possibilities of dimensioning thecomponents favorably, it also avoids initiating the danger of anaccidental breakdown of thyristor 10, due to overloading of thecapacitor battery 11 to an excessive voltage.

According to another embodiment of the invention the transformer 1 ofFIG. 5 is replaced by a currentlimiting transformer so that theaforementioned advantages of such a transformer can be obtained.

The embodiment of FIG. 6 is substantially similar to that of FIG. 1.However, FIG. 6 differs from FIG. 1 in that the shunt circuit formed bythe armature a3 and contact a7 is missing. Instead, an armature a8 andcontact a9 forming part of a contactor 22 forms a shunt between theelectrode terminals or lines x and y when the apparatus is disconnected.According to one embodiment of the invention, the contactor 22 iscontrolled by the control switch 19.

The circuit of FIG. 6 operates as follows.

With the control switch open relay 15 and contactor 22 are unenergized.In that condition, the armatures a1, a2 and a8 contact the contacts a4,a5 and a9. Thyristor 10 is non-conductive or blocked. The capacitorbattery 11 charges over resistance 3 and armature a1 resting againstcontact a4. Current is supplied by the bridge 6. Placing of the gun 16on workpiece 17 after charging is completed, and closing control switch19, energizes contactor 22 and opens the switch formed by armature a8and contact a9. This removes the shunt across the plugs or terminals xand y. This action also energizes relay 15. Thus, armature a2 contactsthe contact a6 and ignites the thyristor 10. The capacitor battery 11discharges through stud 16a so as to weld this stud to the workpiece 17.

If, accidently, the thyristor 10 remains conductive after switch 19 hasopened, or if thyristor l0 accidently becomes conductive while thecapacitor battery is being charged, the current from the bridge 6 flowsthrough the fuse 2, the armature al, the contact a4, the closed elementsa8 and a9 and resistance 3 to the negative pole of the bridge. Thus, thecapacitor battery cannot be charged at all and no voltage is formedbetween the electrode plugs or terminals x and y. If the capacitorbattery 11 is already completely or partly charged when the thyristor 10accidently becomes conductive, it dischargesthrough the armature a8 andcontact a9 without forming a voltage between the electrode lines x andy.

The armature a8 and contact a9, while they must carry the current fromthe bridge 6 and the capacitor battery 11, need not switch thesecurrents. Therefore, they need be constructed only to carry thesecurrents. Of course, care must be taken to assure that when the relay 15trips, the switch formed-by armature a8 and contact a9 is open beforethe thyristor becomes conductive.

FIGS. 1 to 6 illustrate the capacitor bank or battery 11 as a singlecapacitor. However, it should be understood that the capacitor in thefigures represents the battery or bank.

FIGS. 1 to 6 also illustrate the welding gun as having an electrode 16b.This, in effect, constitutes the legs of the welding gun which serve forclosing the circuit to the switch 19. The welding electrode per se iscomposed of the electrode 16a and the stud which contacts the electrode17. 7

While embodiments of the invention have been described in detail, itwill be obvious to those skilled in the art that the invention may beembodied otherwise without departing from its spirit and scope.

What is claimed is:

1. A stud welding apparatus, comprising capacitor means, charging meanscoupled to said capacitor means for charging said capacitor means,welding means, electronic switch means connecting said welding means tosaid capacitor means and capable of being rendered conductive andnon-conductive for selectively discharging said capacitor means throughsaid welding means, control means coupled to said switch means andvariable between a first state and a second state for actuating saidelectronic switch means in the first state so as to render said switchmeans conductive and initiate welding, and protective means responsiveto the conductivity of said switch means and the state of said controlmeans and coupled to said capacitor means for reducing the voltageacross said capacitor means when said switch means is conductive andwhen said control means is in the second state.

2. An apparatus as in claim 1, wherein said protective means includesvoltage abating means coupled to said charging means for reducing thevoltage applied by said charging means across said capacitor means.

3. An apparatus as in claim 2, wherein said protective means includesshunting means responsive to said switch means being conductive and saidcontrol means being in the second state for forming a shunt path throughwhich said capacitor means can discharge.

4. An apparatus as in claim 2, wherein said protective means includescircuit means for forming a current path across said supply unit andincluding said electronic switch means, said circuit means including acutoff device for responding to current through said circuit means, saidcutoff device being connected to said capacitor means for cutting offthe current to said capacitor means when said current device is cut off.

5. A stud welding apparatus, comprising welding current generating meansincluding capacitor means, charging means coupled to said capacitormeans for charging said capacitor means, welding means, said generating.means including electronic switch means connecting said capacitor meansto said welding means and capable of being rendered conductive andnonconductive for selectively discharging said capacitor means throughsaid welding means, control means coupled to said switch means andvariable between a first state and a second state for actuating saidswitch means in the first state so as to render said switch meansconductive and initiate welding, and protective means responsive to theconductivity of said switch means and the state of said control meansand coupled to said generating means for reducing the voltage acrosssaid capacitor means when said switch means is conductive and when saidcontrol means is in the second state.

6. An apparatus as in claim 5, wherein said protective means includesvoltage abating means coupled to said charging means for reducing thevoltage applied by said charging means across said capacitor means.

7. An apparatus as in claim 6, wherein said protective means includesshunting means responsive to said switch means being conductive and saidcontrol means being in the second state for forming a shunt path throughwhich said capacitor means can discharge.

8. An apparatus as in claim 6, wherein said protective means includescircuit means for forming a current path across said supply unit andincluding said electronic switch means, said circuit means including acutoff device for responding to current through said circuit means saidcutoff device being connected to said capacitor means for cutting offthe current to said capacitor means when said current device is cut off.

9. An apparatus as in claim 6, wherein said protective means includessensor means sensing the voltage applied to said welding means, safetyswitch means responsive to said control means being in the second stateand the existence of a predetermined voltage at said welding means forshunting said charging means, said safety switch means including adisconnect element responsive to current through said safety switch fordisconnecting said charging means from said capacitor means.

10. An apparatus as in claim 9, wherein said safety switch meansincludes a thyristor, said sensor means being connected to saidthyristor for turning said thyristor on when said sensor means senses apotential greater than a predetermined potential, said sensor meanssensing a potential when said electronic switch means is conductive,said control means disconnecting said thyristor from said sensor meansin the first state.

11. An apparatus as in claim 10, wherein protective means includesshunting means responsive to said switch means being conductive and saidcontrol means being in the second state for forming a shunt path throughwhich said capacitor means can discharge.

12. An apparatus as in claim 11, wherein said charging means includes aseries resistance, and wherein said path includes said seriesresistance.

13. An apparatus as in claim 12, wherein said protective means furtherincludes overload means responsive to the voltage across said capacitormeans and including comparator means for applying a signal to saidsafety switch means when the voltage across said capacitor means exceedsa predetermined value determined by said capacitor means.

14. An apparatus as in claim 10, wherein said safety switch means formsa shunt path across said charging means, said electronic switch meansbeing outside of said path.

15. An apparatus as in claim 10, wherein said protective means furtherincludes overload means responsive to the voltage across said capacitormeans and including comparator means for applying a signal to saidsafety switch means when the voltage across said capacitor means exceedsa predetermined value determined by said capacitor means.

16. An apparatus as in claim 9, wherein said cutoff element is a fuse.

17. An apparatus as in claim 16, wherein said protective means furtherincludes overload means responsive to the voltage across said capacitormeans and including comparator means for applying a signal to saidsafety switch means when the voltage across said capacitor means exceedsa predetermined value determined by said capacitor means.

18. An apparatus as inclaim 9, wherein said safety switch means forms ashunt path across said charging means, said electronic switch meansbeing outside of said path. .1

19. An apparatus as in claim 18, wherein protective means includesshunting means responsive to said switch means being conductive and saidcontrol means being in the second state for forming a shunt path throughwhich said capacitor means can discharge.

20. An apparatus as in claim 9, wherein said protective means furtherincludes overload means responsive to the voltage across said capacitormeans and including comparator means for applying a signal to saidsafety switch means when the voltage across said capacitor means exceedsa predetermined value determined by said capacitor means.

21. An apparatus as in claim 5, wherein said protective means includesan auxiliary switch connected across said welding means and responsiveto the state of said control means for shunting said welding means whensaid control means is in the second state.

22. A stud welding apparatus as in claim 21, wherein said protectivemeans includes circuit means for forming a current path across saidcharging means and including said electronic switch means, said circuitmeans including a cutoff device for responding to current through saidcircuit means, said cutoff device being connected tosaid capacitor meansfor cutting off the current to said capacitor means when said cut offdevice is cut off;

23. An apparatus as in claim 5, wherein said charging means includesacurrent-limiting transformer and a rectifier circuit. t

24. An apparatus as in claim 23 wherein said protective means includesvoltage abating means coupled to said charging means for reducing thevoltage applied by said charging means across said capacitor means.

-25. An apparatus as in claim 24, wherein said protective means includesshunting means responsive to said switch means being conductive and saidcontrol means being in the second state for forming a shunt path throughwhich said capacitor means can discharge.

26. An apparatus as in claim 23, wherein said protective means includessensor means sensing the voltage applied to said welding means, safetyswitch means, responsive to said control means being in the second stateand the existenceof a predetermined voltage at said welding means forshunting said charging 'means, said safety switch means including adisconnect element responsive to current through said safety switch fordisconnecting said charging means from said capacitor means.

27. An apparatus as in claim 23, wherein said safety switch meansincludes a thyristor, said sensor means being connected to saidthyristor for turning said thyristor on when said sensor means senses apotential greater than a predetermined potential, said sensor meanssensing a potential when said electronic switch means is conductive,said control means disconnecting said thyristor from said sensor meansin the first state.

28. An apparatus as in claim 23, wherein said charging means includes aseries resistance, and wherein said path includes said seriesresistance.

29. An apparatus as in claim 23, wherein said transformer includes aprimary winding .anda secondary winding loosely coupled with saidprimary winding and having a core to form a leakage reactivetransformer.

1. A stud welding apparatus, comprising capacitor means, charging meanscoupled to said capacitor means for charging said capacitor means,welding means, electronic switch means connecting said welding means tosaid capacitor means and capable of being rendered conductive andnon-conductive for selectively discharging said capacitor means throughsaid welding means, control means coupled to said switch means andvariable between a first state and a second state for actuating saidelectronic switch means in the first state so as to render said switchmeans conductive and initiate welding, and protective means responsiveto the conductivity of said switch means and the state of said controlmeans and coupled to said capacitor means for reducing the voltageacross said capacitor means when said switch means is conductive andwhen said control means is in the second state.
 2. An apparatus as inclaim 1, wherein said protective means includes voltage abating meanscoupled to said charging means for reducing the voltage applied by saidcharging means across said capacitor means.
 3. An apparatus as in claim2, wherein said protective means includes shunting means responsive tosaid switch means being conductive and said control means being in thesecond state for forming a shunt path through which said capacitor meanscan discharge.
 4. An apparatus as in claim 2, wherein said protectivemeans includes circuit means for forming a current path across saidsupply unit and including said electronic switch means, said circuitmeans including a cutoff device for responding to current through saidcircuit means, said cutoff device being connected to said capacitormeans for cutting off the current to said capacitor means when saidcurrent device is cut off.
 5. A stud welding apparatus, comprisingwelding current generating means including capacitor means, chargingmeans coupled to said capacitor means for charging said capacitor means,welding means, said generating means including electronic switch meansconnecting said capacitor means to said welding means and capable ofbeing rendered conductive and non-conductive for selectively dischargingsaid capacitor means through said welding means, control means coupledto said switcH means and variable between a first state and a secondstate for actuating said switch means in the first state so as to rendersaid switch means conductive and initiate welding, and protective meansresponsive to the conductivity of said switch means and the state ofsaid control means and coupled to said generating means for reducing thevoltage across said capacitor means when said switch means is conductiveand when said control means is in the second state.
 6. An apparatus asin claim 5, wherein said protective means includes voltage abating meanscoupled to said charging means for reducing the voltage applied by saidcharging means across said capacitor means.
 7. An apparatus as in claim6, wherein said protective means includes shunting means responsive tosaid switch means being conductive and said control means being in thesecond state for forming a shunt path through which said capacitor meanscan discharge.
 8. An apparatus as in claim 6, wherein said protectivemeans includes circuit means for forming a current path across saidsupply unit and including said electronic switch means, said circuitmeans including a cutoff device for responding to current through saidcircuit means, said cutoff device being connected to said capacitormeans for cutting off the current to said capacitor means when saidcurrent device is cut off.
 9. An apparatus as in claim 6, wherein saidprotective means includes sensor means sensing the voltage applied tosaid welding means, safety switch means responsive to said control meansbeing in the second state and the existence of a predetermined voltageat said welding means for shunting said charging means, said safetyswitch means including a disconnect element responsive to currentthrough said safety switch for disconnecting said charging means fromsaid capacitor means.
 10. An apparatus as in claim 9, wherein saidsafety switch means includes a thyristor, said sensor means beingconnected to said thyristor for turning said thyristor on when saidsensor means senses a potential greater than a predetermined potential,said sensor means sensing a potential when said electronic switch meansis conductive, said control means disconnecting said thyristor from saidsensor means in the first state.
 11. An apparatus as in claim 10,wherein protective means includes shunting means responsive to saidswitch means being conductive and said control means being in the secondstate for forming a shunt path through which said capacitor means candischarge.
 12. An apparatus as in claim 11, wherein said charging meansincludes a series resistance, and wherein said path includes said seriesresistance.
 13. An apparatus as in claim 12, wherein said protectivemeans further includes overload means responsive to the voltage acrosssaid capacitor means and including comparator means for applying asignal to said safety switch means when the voltage across saidcapacitor means exceeds a predetermined value determined by saidcapacitor means.
 14. An apparatus as in claim 10, wherein said safetyswitch means forms a shunt path across said charging means, saidelectronic switch means being outside of said path.
 15. An apparatus asin claim 10, wherein said protective means further includes overloadmeans responsive to the voltage across said capacitor means andincluding comparator means for applying a signal to said safety switchmeans when the voltage across said capacitor means exceeds apredetermined value determined by said capacitor means.
 16. An apparatusas in claim 9, wherein said cutoff element is a fuse.
 17. An apparatusas in claim 16, wherein said protective means further includes overloadmeans responsive to the voltage across said capacitor means andincluding comparator means for applying a signal to said safety switchmeans when the voltage across said capacitor means exceeds apredetermined value determined by said capacitor means.
 18. An apparatusas in claim 9, wherein said safety switch means Forms a shunt pathacross said charging means, said electronic switch means being outsideof said path.
 19. An apparatus as in claim 18, wherein protective meansincludes shunting means responsive to said switch means being conductiveand said control means being in the second state for forming a shuntpath through which said capacitor means can discharge.
 20. An apparatusas in claim 9, wherein said protective means further includes overloadmeans responsive to the voltage across said capacitor means andincluding comparator means for applying a signal to said safety switchmeans when the voltage across said capacitor means exceeds apredetermined value determined by said capacitor means.
 21. An apparatusas in claim 5, wherein said protective means includes an auxiliaryswitch connected across said welding means and responsive to the stateof said control means for shunting said welding means when said controlmeans is in the second state.
 22. A stud welding apparatus as in claim21, wherein said protective means includes circuit means for forming acurrent path across said charging means and including said electronicswitch means, said circuit means including a cutoff device forresponding to current through said circuit means, said cutoff devicebeing connected to said capacitor means for cutting off the current tosaid capacitor means when said cut off device is cut off.
 23. Anapparatus as in claim 5, wherein said charging means includes acurrent-limiting transformer and a rectifier circuit.
 24. An apparatusas in claim 23 wherein said protective means includes voltage abatingmeans coupled to said charging means for reducing the voltage applied bysaid charging means across said capacitor means.
 25. An apparatus as inclaim 24, wherein said protective means includes shunting meansresponsive to said switch means being conductive and said control meansbeing in the second state for forming a shunt path through which saidcapacitor means can discharge.
 26. An apparatus as in claim 23, whereinsaid protective means includes sensor means sensing the voltage appliedto said welding means, safety switch means responsive to said controlmeans being in the second state and the existence of a predeterminedvoltage at said welding means for shunting said charging means, saidsafety switch means including a disconnect element responsive to currentthrough said safety switch for disconnecting said charging means fromsaid capacitor means.
 27. An apparatus as in claim 23, wherein saidsafety switch means includes a thyristor, said sensor means beingconnected to said thyristor for turning said thyristor on when saidsensor means senses a potential greater than a predetermined potential,said sensor means sensing a potential when said electronic switch meansis conductive, said control means disconnecting said thyristor from saidsensor means in the first state.
 28. An apparatus as in claim 23,wherein said charging means includes a series resistance, and whereinsaid path includes said series resistance.
 29. An apparatus as in claim23, wherein said transformer includes a primary winding and a secondarywinding loosely coupled with said primary winding and having a core toform a leakage reactive transformer.